When Forest uses a Surface as a scattering area, it extracts a "virtual" spline from closed edges of the mesh, and this is used as a Spline Area for several operations, including Size/Edge boundary checking, Area Falloffs and others.
Depending of the surface topology, this spline is more or less adequate to be used as Spline Area. In some particular cases, it could result in some not ideally working shapes (even lead to self-intersections, that may create an endless loop in one of the Max functions which process it).
If similar problem occurs, it can be fixed manually:
1.- Create a Copy of the surface object & convert it to Editable Poly if needed.
3.- Under Polygon Selection mode select all faces used for scattering (sometimes the Select By Angle from Ribbon could be very handy).
4.- Having Ctrl key pressed switch to Edge Selection mode.
5.- Than hit under Edit Edges -> Create Shape. It creates a new Editable Spline object to be used with Forest.
6.- Select the Forest object, under Areas rollout Add a new Spline Area and assign the shape created before.
7.- Select the Surface Area and turn it Off (Forest will not generate items from it).
By following these steps it'll give you nice and clean edges on any Surface(s).